The present invention describes an apparatus for the deposition and/or removal of thin layers of materials on or from a substrate such as a silicon wafer.
A number of advanced deposition and etching processes make use of discrete steps for the formation of a thin film or the etching of deep structures. These steps are preferably performed in one process chamber (either batch or single wafer).
By way of example, Atomic Layer deposition (ALD—also known as atomic layer epitaxy) is a chemical vapor deposition process in which self-limiting surface reactions produce extremely conformal coatings. There is a growing requirement to increase equipment productivity for ALD and similar processes for example to form very thin layers high dielectric constant (k) films such as HfO2 or alumina. For the ALD of a layer such as Aluminum oxide a reactant gas containing Aluminum such as trimethyl aluminum Al(CH3)3 is firstly introduced into a chamber containing the substrate(s) such that a monolayer of this gas adheres to and in many cases saturates the surface. A purge is then necessary by pumping only or with an inert (to this process) gas such as nitrogen or hydrogen to remove all but this monolayer of gas from the chamber. Then an oxidizer, such as water vapor, is flowed into the chamber and a surface reaction with the trimethyl aluminum takes place forming a monolayer of aluminum oxide. Excess oxidizer and reaction by-products are then pumped away and the chamber purged and the cycle repeated until the desired film thickness achieved.The cycle time therefore=Exposure A+Purge A+Exposure B+Purge B
Purge is used throughout to mean the sufficient removal or dilution of the reactant. This may be achieved by pumping alone or by pumping and flowing an inert (Purge) gas or any sequence of the two as is well known in the field.
Where A and B represent the precursors from which the deposited film is derived. This is example is given simply by way of example and a review of the field will give many further examples of such step-wise deposition processes.
Whilst this binary process produces high quality films and is relatively insensitive to other process parameters it is very slow, as the layers are built up monolayer at a time.
The necessary exposure time is very short so a known attractive approach to increasing the speed of film growth is to decrease exposure times. Whilst exposure times may be reduced to very short periods e.g. sub 1 second, it is a critical requirement that the process chamber is adequately purged after the precursor exposure steps.
Presently there is no method of reducing the purge times to those as short as the exposure times and in an otherwise optimized process perhaps 75% of the total cycle time is spent (unproductively) purging the chamber.